Clathrates are non-stoichiometric crystalline compounds consisting of two molecular species, where one species physically entraps the other within a cage-like structure. The species forming the cage-like structure is commonly referred to as the host, while the caged component is commonly referred to as the guest. When the cage-like structure is made up of water molecules bonded together, the crystalline compounds formed are known as clathrate hydrates or gas hydrates.
In gas hydrates, the host-lattice is created by water molecules connected together through hydrogen bonding. The guest molecule is held in place inside cavities of the hydrogen-bonded water molecules, and the lattice is stabilized by van der Weals forces between host and guest molecules without chemical bonding between the host-lattice and guest molecule. The host-lattice is thermodynamically unstable without the presence of a guest molecule in the cavity, and without the support of the trapped molecules, the lattice structure of gas hydrates will collapse into conventional ice crystal structures or liquid water. Most low molecular weight gases, including O2, H2, N2, CO2, CH4, H2S, Ar, Kr, and Xe as well as some higher hydrocarbons and freons, will form hydrates at suitable temperatures and pressures. The storage capacity for gas in these structures is considerable, and gas hydrates are attractive options for the storing and transportation natural gas and other gases as an alternative to liquefying or compression.
Formation and decomposition of gas hydrates are first order phase transitions rather than chemical reactions, and generally the typical mechanism of hydrate formation in a pure water-gas system proceeds by water molecules first forming clusters by hydrogen bonding in the liquid phase, and then proceeding to cluster and occlude gas until a critical size of the clusters is reached. As is known in the art, hydrate nucleation refers to the process where hydrate nuclei, grow and disperse until they attain the critical size. If the size of the nuclei is less than the critical size, the nuclei are unstable and may continue to grow or break in the aqueous solution. If the growing nuclei reach the critical size they then becomes stable, which leads to the formation of hydrate crystals. This period from when the hydrate nuclei are forming and dissolving to the time when the nuclei reach the critical size is called the induction time. Induction times exceeding 6 hours are not uncommon. Because the water and hydrate forming gas must be maintained at the temperature and pressure condition for hydrate formation during the induction time, this requirement can generate large capital costs and is of paramount importance.
As a result, reducing the induction time and increasing the rate of hydrate production are significant areas of interest in hydrate forming technologies. Generally speaking, efforts concentrate on improvements in the two conditions that need to be satisfied in order to produce a high rate of hydrate formation: (1) a good mixing of the hydrate-forming gas and water, and (2) an effective cooling for removing the heat released by the exothermic hydrate formation. A primary means of optimizing these conditions has been through utilization of mechanisms generally termed water-spraying reactors.
In a water-spraying reactor, liquid water is sprayed in a continuous phase of a hydrate-forming gas or gases. Typically, water is sprayed downward by an atomizer into the hydrate-forming gas, which is confined in a vertically oriented cylindrical cell with its side wall being cooled from the outside. See e.g., U.S. Pat. No. 5,536,893, issued Jun. 16, 1996, issued to Gudmundsson, among others. This methodology has been successful in the production of gas hydrates, however the basic approach has disadvantages in both effective cooling and mixing. The cooling mechanism is mainly based on spray-to-wall heat transfer, and this mechanism may become increasingly ineffective with an increase in the reactor size. Additionally, in a reactor equipped with multiple spray nozzles, many of the water sprays formed may be away from the wall of the reactor, and may be minimally cooled.
Another method of cooling is utilization of impingement heat transfer from sprayed water to the surface of a cooled, highly heat-conductive slab, so that heat released by gas hydrate formation on the slab is directly removed by heat conduction into the slab. See Matsuda et al., “Hydrate Formation Using Water Spraying onto a Cooled Solid Surface in a Guest Gas,” AlChE Journal, Vol. 52, No. 8 (August 2006). This approach can suffer from formation of a gas hydrate layer on the metal block surface, acting to hindering the heat flow into the block.
An alternative approach involves a water spray, collection of the water accumulated at the bottom of the reactor, and pumping the accumulated water to an external heat exchanger to be cooled and reintroduced to the reactor to be sprayed again. See e.g., U.S. Pat. No. 6,653,516, issued Nov. 25, 2003, issued to Yoshikawa, et al. The cooled circulating water compensates for the heat released by the hydrate formation, and the rate of water circulation is varied in proportion to the capacity of the reactor. However, obstructing the water-circulation loop due to hydrate formation is a significant risk, and the water flow rate through the loop may need to be maintained at an excessively high-level as compared to the rate of water consumption due to the hydrate formation. This increases the pumping power requirement.
The foregoing methods also introduce water and the hydrate-forming gas into the reactor separately, and rely on mixing within the reactor itself in order to generate sufficient contact between the water and gas. Various methods have been utilized to improve the mixing process and thereby increase the rate of hydrate formation, including stirring mechanisms and other similar processes that apply shear forces to gas bubbles in the mixture. These processes act to decrease bubble size and increase the contact between water in the reactor and the hydrate-forming gas, however the means by which the shear force is generated and applied adds additional mechanical and operational complication to the reactor.
Efforts to increase the rate of methane hydrate formation with a spray of water and dissolved methane have also been investigated, in order to obtain memory effects and reduce induction time. See '516 to Yoshikawa, et al; See also Holder et al., “Formation of gas hydrates from single-phase aqueous solutions,” Chemical Engineering Science 56 (2001); See also Lee et al., “Methane—ethane and methane—propane hydrate formation and decomposition on water droplets,” Chemical Engineering Science, Vol. 60 (2005). However, many hydrate forming gases such as methane have extremely low solubility in water and the production of a single-phase system consisting of liquid water and a dissolved hydrate-forming gas presents additional difficulties not encountered when hydrate formation can be effectively accomplished using separate phases of liquid water and a gaseous hydrate-former.
It would be advantageous to provide a process whereby the exothermic heat of gas hydrate formation could be more effectively removed without reliance on contact between reactor walls or other heat transfer surfaces, and without reliance on water sub-cooling systems generating significant parasitic loads. Further, it would be advantageous to provide a process whereby water and a gas-forming hydrate could be introduced into a hydrate-forming reactor as a mixture, so that reliance on shear forces or water-gas intermingling for mixing within the reactor itself could be mitigated. Further, it would be advantageous to provide a process compatible with the water-gas mixture entering the reactor in a two-phase state, so that difficulties associated with producing a single-phase system consisting of liquid water with dissolved hydrate could be avoided. Further, it would be advantageous to provide a process whereby some degree of exothermic heat removal could be accommodated by the thermodynamic response of the mixture to the hydrate reactor conditions, and where that response could simultaneously aid in further contact between the water and the hydrate-forming gas of the mixture.
Accordingly, it is an object of this disclosure to provide a method and apparatus for the formation of gas hydrates where the exothermic heat of gas hydrate formation can be more effectively removed without reliance on contact between reactor walls or other heat transfer surfaces.
Further, it is an object of this disclosure to provide a method and apparatus for the formation of gas hydrates where the exothermic heat of gas hydrate formation can be more effectively removed without reliance on circulating water sub-cooling systems generating significant parasitic loads.
Further, it is an object of this disclosure to provide a method and apparatus for the formation of gas hydrates where water and a gas-forming hydrate can be introduced into a hydrate-forming reactor as a mixture, so that reliance on shear forces for mixing within the reactor itself can be mitigated.
Further, it is an object of this disclosure to provide a method and apparatus for the formation of gas hydrates compatible with the water-gas mixture entering the reactor in a two-phase state, so that difficulties associated with producing a single-phase system consisting of liquid water with dissolved hydrate can be avoided.
Further, it is an object of this disclosure to provide a method and apparatus for the formation of gas hydrates where some degree of exothermic heat removal can be accommodated by the thermodynamic response of the mixture to the hydrate reactor conditions, and where that response can simultaneously aid in further contact between the water and the hydrate-forming gas of the mixture.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.